1. Field of the Invention
The present invention relates to an acrylic elastomer and its composition and more particularly to an acrylic elastomer capable of giving stable compression set characteristics and its composition.
2. Related Art
Carboxyl group-containing acrylic elastomers, particularly acrylic elastomers in which the carboxyl groups of butenedioic acid monoalkyl ester act as cross-linking sites, are characterized by giving a cross-linking products with a distinguished compression set, free from any fear of corrosion of metal due to their non-halogenic properties per se by cross-linking reaction with a polyamine compound.
Such a rubbery material has good steam vulcanization characteristics and is effectively used in the field of strict requirements for compression set characteristics such as the field of sealing materials such as gaskets, oil seals, O-rings, etc. (JP-A-11-140264 and 11-269336) or the field of hose materials (JP-A-6-99515). However, vulcanization characteristics and compression set characteristics of such a rubbery material are much fluctuated and neither satisfactory compression set characteristics nor satisfactory vulcanization characteristics can be obtained, depending upon cases.
Furthermore, carboxyl group-containing acrylic elastomers, particularly butenedioic acid monoalkyl ester, are less copolymerizable with other acrylic monomers. For example, in case of maleic acid monoalkyl ester the esters undergo copolymerization only in such an amount as ⅓-¼ times the necessary one for cross-linking site formation, and thus must be charged in an increased amount, resulting in a large amount of remaining unreacted monomers.